The present invention relates to turbine components having a substrate formed from a ceramic material, such as a monolithic ceramic or a composite ceramic material, and a thermal barrier coating made from ceramic materials.
Gas turbine engines are well developed mechanisms for converting chemical potential energy, in the form of fuel, to thermal energy and then to mechanical energy for use in propelling aircraft, generating electrical power, pumping fluids, etc. At this time, the major available avenue for improved efficiency of gas turbine engines appears to be the use of higher operating temperatures. However, the metallic materials used in gas turbine engines are currently very near the upper limits of their thermal stability. In the hottest portion of modern gas turbine engines, metallic materials are used at gas temperatures above their melting points. They survive because they are air cooled. But providing air cooling reduces engine efficiency.
Accordingly, there has been extensive development of thermal barrier coatings for use with cooled gas turbine aircraft hardware. By using a thermal barrier coating, the amount of cooling air required can be substantially reduced, thus providing a corresponding increase in efficiency.
Such coatings are invariably based on ceramic. Mullite and alumina have been proposed, but zirconia is the current material of choice. Zirconia must be modified with a stabilizer to prevent the formation of the monoclinic phase. Typical stabilizers include yttria, calcia, ceria, and magnesia.
Zirconia based ceramics are resistant to water attack. This is critical for land-based gas turbine applications, since the coatings are exposed at high temperatures for much longer than they are in aeroengine applications. Thus, corrosion of the thermal barrier coating can become a problem—indeed it is known to be a problem for silica-based thermal barrier coatings and alumina-based thermal barrier coatings. Steam is often injected into the combustor of land-based gas turbines to reduce nitric oxide formation, which exacerbates the water attack issue.
Despite the success with thermal barrier coatings, there is a continuing desire for improved coatings which exhibit superior thermal insulation capabilities, especially those improved in insulation capabilities when normalized for coating density. Weight is always a critical factor when designing gas turbine engines, particularly rotating parts. Ceramic thermal barrier coatings are not load supporting materials, and consequently they add weight without increasing strength. There is a strong desire for a ceramic thermal barrier material which adds the minimum weight while providing the maximum thermal insulation capability. In addition, there are the normal desires for long life, stability and economy.